Magnetic bubble domain devices are well known in the art. There are two methods by which bubbles are propagated in bubble devices; one is the usual field access type and the second is the current (or conductor) access type. The most familiar mode of operating a magnetic bubble device is termed the "field-access" mode. In this mode, a pattern of magnetically soft elements (such as permalloy or ion-implanted contiguous disks) is formed in a plane adjacent a layer of material in which the bubbles are moved. A magnetic field is generated in the plane of the layer and the field caused to reorient to incrementally-offset radial positions cyclically in the plane. Each element is so shaped that various portions thereof respond to the in-plane field to generate pole-patterns which change as the field precesses. The configuration of adjacent elements sets up a sequence of travelling potential wells in the layer which causes bubble movement.
In current access devices, the necessary potential wells are provided by a set of conductor patterns in which polyphase, usually two or three phase, currents are transmitted. The conductors are typically formed in multiple layers, insulated from one another and driven in a two or three phase manner. An example of such a device is described in U.S. Pat. No. 3,460,116.
Various types of magnetic bubble domain device architectures are known in the prior art, including the major loop minor loop configuration. The major loop/minor loop configuration, such as described in U.S. Pat. No. 3,618,054, consists of a plurality of first recirculating "minor" channels and a second "major" channel.
Bubble detection normally involves the expansion of the bubble into a long stripe domain for the purpose of increasing the magnetic flux available for sensing to achieve adequate signal level. In field access permalloy devices, the expansion is accomplished by using a number of chevron stacks which act both to propagate and simultaneously expand the bubble perpendicular to the propagation direction. It is thus possible to obtain very long stripe domains by gradual expansion over a number of field cycles in as many chevron stacks. The cost is paid only in large detector areas.
In current access devices, the bubble expansion is performed in a current driven conductor expander usually in the form of a single conductor loop (such as in U.S. Pat. No. 3,564,518), or a dual conductor gradual expander configuration. In both configurations, large currents are applied for the period required to expand the bubble. The result is that only small expansion lengths can be achieved (practically around 100 .mu.m in 8 .mu.m period devices operating at 100 kHz). The reason is that the power dissipation increases as the square of the expansion length. Furthermore, the prior art configurations do not allow for consecutive bit detection if the expansion time is longer than the stepping rate of the circuit (inverse of the frequency of operation).
In ion-implanted contiguous disk devices, the bubble detection is performed in current-assisted stretcher detector which is composed of a hair-pin conductor loop and a thin permalloy bar located along the loop. A bubble propagating in the ion-implanted track is stretched as it arrives at a predetermined location inside the loop by applying a sufficiently large current in the conductor loop to reduce the local bias field to below the bubble strip out field. Due to the finite mobility of the domain wall motion, the stretching of the bubble requires a fairly large fraction of the field cycle. In devices reported by T. J. Nelson and R. Wolf, in a paper entitled "Design of Bubble Device Element Employing Ion-Implanted Propagation Patterns", presented orally at the Intermag Conference in Boston, Mass. in May 1980, a stretcher 100 .mu.m long requiring 2.5 .mu.sec. stretch pulse with 70 to 100 ma pulse amplitude was described. The sense signal was about 1 to 2 m volts. To obtain larger signals or to operate at higher frequency, the pulse width to period ratio must be increased to allow for longer stretch time. This leads to larger power dissipation in the detector which might then exceed its power handling capacity resulting in device failure.
Prior to the present invention, there has not been a stretcher detector capable of operating at high frequency with low power dissipation.